The present invention relates generally to (Light Emitting Diode) LED array type light sources, and more particularly, to an LED array that includes LEDs connected in series, and having an active shunt arrangement to enable one or more failed LEDs to be bypassed, thereby averting failure of the entire LED array or an entire string of series-connected LEDs within the LED array.
LED array type light sources are currently in widespread use in a variety of different signaling and lighting applications, such as image sensors for facsimile machines and the like, and LED-based luminaires and light-engine products. From the standpoint of drive electronics, it is usually advantageous to connect all of the LEDs in series, since this results in a relatively high-voltage, low-current load, which is usually more economical to drive. For example, a 50 V/1 A load is usually more economical to drive than is a 5 V/10 A load. However, while usually advantageous from the standpoint of the drive electronics, this approach has a major drawback. More particularly, when all of the LEDs are connected in series, the failure (i.e., open circuit condition) of any one of the LEDs renders the entire LED array inoperative, i.e., a failure of any one of the series-connected LEDs results in a failure of the entire string of series-connected LEDs that includes the failed LED. For this reason, most present-day LED array type light sources incorporate a combination of series-connected and parallel-connected strings of LEDs to avoid a failure of the entire LED array upon failure of a single LED within the array. However, this solution is undesirably complex and compromises drive efficiency. Moreover, the light pattern and/or light output of the LED array is adversely affected by failure of a single LED, since an entire string of series-connected LEDs within the overall LED array is still subject to failure upon failure of a single LED within that string.
PCT Application Publication Number WO 97/29320 having an international publication date of Aug. 14, 1997, discloses a "Flight Obstacle Light" that includes an LED array that has four branches of series-connected LEDs, each of which can be located on separate circuit boards. Further, a zener diode is connected in parallel with every LED, whereby if a particular LED fails, then the current will be shunted through the associated zener diode, thus avoiding failure of the entire branch of'series-connected LEDs that includes the failed LED. Although this solution is simple, and effectively prevents failure of an entire string or branch of series-connected LEDs upon failure of a single LED within that string or branch, it suffers from a significant drawback. More particularly, the zener diodes are passive shunts which will generate (dissipate) an undesirable amount of heat while in operation.
Based on the above and foregoing, there presently exists a need in the art for an LED array that overcomes the above-described drawbacks and shortcomings of the presently available technology. The present invention fulfills this need in the art.